Method for Producing a Piezoelectric Multilayer Component and a Piezoelectric Multilayer Component

ABSTRACT

A method for producing a piezoelectric multilayer component is disclosed. Piezoelectric green films and electrode material are provided, arranged alternately on top of one another and sintered. The electrode material is provided with a PbO-containing coating and/or PbO is mixed into the electrode material.

This patent application is a divisional of U.S. patent application Ser.No. 13/510,888 filed on Jul. 31, 2012 titled “Method for Producing aPiezoelectric Multilayer Component and a Piezoelectric MultilayerComponent,” which is a national phase filing under section 371 ofPCT/EP2011/050847, filed Jan. 21, 2011, which claims the priority ofGerman patent application no. 10 2010 005 403.8, filed Jan. 22, 2010,each of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

A method for producing a piezoelectric multilayer component is provided,in which a stack of piezoelectric green films and layers of electrodematerial is formed and sintered.

SUMMARY

A piezoelectric multilayer component comprising a stack of sinteredpiezoelectric layers and inner electrodes arranged between them isfurthermore provided. Such a multilayer component is, for example, apiezo actuator, which can be used to operate an injection valve in amotor vehicle.

The reliability of piezoelectric multilayer components depends on thecontrol over cracks possibly occurring during their production. Suchcracks may, for example, occur during thermal processes such assintering, metallization and soldering or during the polarization, sinceelastic stresses are formed owing to different expansions in variousregions of the component. Such so-called relaxation cracks or polingcracks may furthermore change direction, extend perpendicularly to theelectrodes and therefore for example bridge two electrodes, which leadsto a short circuit and failure of the component.

In one aspect, the present invention provides a method for producing apiezoelectric multilayer component which has increased reliability. Inanother aspect, the present invention provides a piezoelectricmultilayer component having increased reliability. This object isachieved by a multilayer component according to patent claim 11.

A method for producing a piezoelectric multilayer component is provided,which comprises the steps:

A) providing piezoelectric green films containing a piezoelectricmaterial,

B) providing an electrode material containing Pd,

C) alternately arranging green films and layers of electrode material inorder to form a stack, and

D) sintering the stack.

At least one layer of electrode material is provided with a coatingwhich contains PbO in method step C) and/or PbO is mixed with theelectrode material in method step B).

With this method, a piezoelectric multilayer component is produced whichcomprises piezoelectric ceramic layers with inner electrodes arrangedbetween them.

The piezoelectric green films may comprise a material which can besintered to form lead zirconate titanate (PZT) ceramic.

“Alternately arranging” may also mean that a layer of electrode materialis not applied on every green film. For example, some piezoelectricgreen films may be arranged above one another without there being anylayers of electrode material between them.

The layers of electrode material form the inner electrodes in thefinished multilayer component, and they may be applied onto the greenfilms as a metal paste by means of a screen printing method.

The electrode material, which contains Pd and may be a metal paste, maycomprise a mixture or an alloy of Ag and Pd or of Cu and Pd. Othermixtures and alloys containing Pd may likewise be envisaged.Furthermore, the electrode material may also be a metal paste containingPd, into which PbO is added in a uniformly distributed way.

The coating containing PbO, which is applied onto the layer of electrodematerial in method step C), may furthermore be applied onto theelectrode material by means of screen printing.

The stack formed in method step C) is subsequently compressed and thenthe green films and the layers of electrode material are sinteredtogether in method step D), so that a multilayer component consisting ofpiezoelectric layers with inner electrodes arranged between them isformed.

Method step D) may furthermore comprise the substeps:

D1) sintering at a temperature of up to 400° C.,

D2) sintering at a temperature which lies in a range of from 400° C. to700° C.,

D3) sintering at a temperature of more than 700° C., an intermediatephase containing PbPdO₂ being formed between the electrode material andthe piezoelectric material in substep D2).

The temperature in substep D3) may, for example, be up to 1200° C.During the sintering, reactions take place between the PbO, which iscontained in the coating and/or in the electrode material, and the Pdwhich is contained in the electrode material, in which case the PbPdO₂is formed. The intermediate phase containing PbPdO₂ may thus, forexample, be formed where the coating of PbO was applied on the electrodematerial. An intermediate phase consisting of PbPdO₂ may thus be formed,which is present between the piezoelectric material and the electrodematerial, while the coating which contains PbO is substantiallydecomposed. The intermediate phase may, for example, have a thickness ofless than 1 p.m.

The reactions which take place in method step D) may be described asfollows:

PdO in substep D1) is formed by the sintering gas atmosphere, with thePd diffusing out of the electrode material:

Pd+½O₂→PdO

In the subsequent substep D2), the PbPdO₂ intermediate phase is formedfrom the PdO together with the PbO present in the coating and/or in theelectrode material. The coating is substantially decomposed during this:

PdO+PbO→PbPdO₂

Lastly, the PbPdO₂ may be at least partially decomposed into Pd and PbOin substep D3):

PbPdO₂PbO+Pd+½O₂

The metallic Pd may in this case diffuse back into the electrodematerial. The free PbO may at least partially escape into the atmosphereor remain in the piezoelectric material or electrode material.

The PbPdO₂ formed in substep D2) may have a larger volume than PbO andthan the piezoelectric material. Owing to the volume change occurringbetween the substeps, a stress increase may occur in the region betweenthe electrode material and the piezoelectric material, which may lead tosmall cracks, so-called microcracks. These occur where the reactionbetween PbO and Pd has taken place, that is to say in the region inwhich the coating was present on the electrode material before thesintering, i.e. between the electrode material and the piezoelectricmaterial.

During operation of the multilayer component or its polarization, thesemicrocracks may open to form relaxation cracks or poling cracks. Sincethe coating containing PbO on the electrode material limits the regionin which microcracks can form to the region which is directly adjacentto the inner electrodes of the finished multilayer component, the polingand/or relaxation cracks can likewise be restricted to this region. Theregion which is directly adjacent to the inner electrodes of thefinished multilayer component, and to which the formation of themicrocracks is restricted, is also produced when only the electrodematerial contains PbO since in this case as well PbPdO₂ is formed onlyin this region. The formation and the profile of poling and/orrelaxation cracks are therefore deliberately dictated, so thatuncontrolled growth and branching of these cracks transversely to theinner electrodes can be reduced or prevented.

In this method, PbO with which the Pd from the electrode material reactsto form PbPdO₂ is distributed homogeneously on the surface of theelectrode material. In addition or as an alternative, it may also bemixed with the electrode material so that it is also distributedhomogeneously in the electrode material. The effect achievable by thisis that the PbPdO₂ intermediate phase is formed uniformly on the surfaceof the electrode material, between the electrode material and thepiezoelectric material.

The result of this is that an intermediate phase, which has a differentchemical composition to the piezoelectric material and therefore also adifferent mechanical strength and different elastic properties, isformed uniformly on the surface of the electrode material, between theelectrode material and the piezoelectric material. The thermal expansioncoefficients of this intermediate phase and of the piezoelectricmaterial may also be different. The formation of microcracks inprecisely this limited region of the intermediate phase is thus promotedwhen, in substep D3), the sintering process is lengthened and finallyconcluded. In order to ensure the controlled formation of microcracks inthe limited region, substep D2) may be carried out for a longer periodof time compared with substeps D1) and D3).

In the finished multilayer component, there is then a region weakened bymicrocracks on at least one inner electrode, in which for example polingcracks that are restricted to this region can be formed during thepolarization. The crack growth may therefore deliberately be constrainedparallel to the inner electrodes.

Such poling cracks may be formed in a method step E) which followsmethod step D) and in which the sintered stack is poled.

To this end, after the sintering, outer electrodes are applied on thestack and the piezoelectric layers which have been formed from thepiezoelectric green films during the sintering are polarized.

In this case, for example, a DC voltage is applied between neighboringinner electrodes and the stack is heated. In so-called inactive zones inwhich neighboring inner electrodes of different polarity do not overlapin the stack direction, the piezoelectric material does not expand orexpands less than in the active zones in which overlap takes place.Owing to the different expansion of the piezoelectric layers in inactiveand active zones, mechanical stresses are formed which can lead to thecracks during the polarization.

Poling cracks can therefore be formed in method step E). These areformed in the region in which microcracks are already present. Thepoling cracks can be formed from these microcracks. The poling crackstherefore extend parallel to the inner electrodes in the region in whichthe PbPdO₂ intermediate phase was present during the sintering.

This method thus has the effect that the formation of a PbPdO₂intermediate phase can become more efficient. Inner electrodes with orwithout a coating containing PbO may in this case have a thickness offrom 3 to 5 μm, and the layers of piezoelectric material may have athickness of from 30 to 100 μm. Owing to this thickness difference, allof the PbO from the electrode material if it was mixed therewith, orfrom its coating, can be used for the reaction with the Pd and thedeliberate formation of the intermediate phase between the electrodematerial and the piezoelectric material in the method.

The formation of the intermediate phase can in this case be lessdependent on the temperature profile and/or oxygen partial pressureduring the sintering in method step D), since PbO is available in excessfor the formation of the intermediate phase.

The costs for this method can be further reduced by reducing the Pdcontent in the electrode material. The intermediate phase becomesthinner when less Pd is available for the reaction with PbO, so thatfewer microcracks restricted to the region of the intermediate phase arealso formed, which in turn leads to fewer relaxation and/or polingcracks.

A piezoelectric multilayer component is furthermore provided, whichcomprises a stack of sintered piezoelectric layers and inner electrodesarranged between them, wherein a region which comprises poling cracks ispresent on the surface of at least one electrode.

This region may be present on the entire surface of the at least oneinner electrode so that the poling cracks extend parallel to the innerelectrode.

An increased PbO content compared with the piezoelectric layers, and/ortraces of PbPdO₂, may furthermore be present in this region.

A “piezoelectric layer” refers to a section of the stack which comprisesa piezoelectric material and is bounded in the stack direction by twoneighboring inner electrodes. A piezoelectric layer is formed from oneor more piezoelectric sheets, which are arranged above one another alongthe stack direction. For example, a piezoelectric sheet may result froma piezoelectric green film. A piezoelectric layer may also comprise onlya single piezoelectric sheet.

The inner electrode may comprise a material which contains Pd. Thematerial may be selected from a group which comprises mixtures andalloys of Ag and Pd or Cu and Pd.

The piezoelectric layers may comprise a ceramic which contains leadzirconate titanate.

Preferably, outer electrodes are applied onto two opposite outer facesof the component. An outer electrode comprises, for example, a basemetallization burnt in on the stack. The inner electrodes are preferablyconnected alternately to the outer electrodes along the stack directionof the component. To this end, the inner electrodes are, for example,led alternately up to one of the outer electrodes and have a spacingfrom the second outer electrode. In this way, the inner electrodes ofthe same polarity are electrically connected to one another via a commonouter electrode.

The poling cracks may be substantially free of branching. A multilayercomponent which has a high reliability is thereby provided. Owing totheir spatial arrangement, the poling cracks which are substantiallyfree of branching and extend parallel to the at least one innerelectrode cannot lead to a short circuit of the component, since theycannot bridge a plurality of inner electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS

The specified method and the specified multilayer component and theiradvantageous configurations will be explained below with the aid ofschematic figures, which are not true to scale, and with the aid ofexemplary embodiments.

FIG. 1 shows the schematic side view of the multilayer component;

FIG. 2a shows an enlarged detail of the schematic side view of themultilayer component;

FIG. 2b shows another enlarged detail of the schematic side view of themultilayer component;

FIG. 3a shows another enlarged detail of the schematic side view of themultilayer component; and

FIG. 3b shows another enlarged detail of the schematic side view of themultilayer component.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows a schematic side view of a piezoelectric multilayercomponent in the form of a piezo actuator. The component comprises astack 1 of piezoelectric layers 10 arranged above one another and innerelectrodes 20 lying between them. The inner electrodes 20 are formed aselectrode layers. The piezoelectric layers 10 and the inner electrodes20 are arranged above one another.

The piezoelectric layers 10 contain a ceramic material, for example leadzirconate titanate (PZT) or a lead-free ceramic. The ceramic materialmay also contain dopants. The inner electrodes 20 contain, for example,a mixture or an alloy of Ag and Pd or Cu and Pd. The Pd has a proportionby weight of up to 40% in the electrode material.

In order to produce the stack 1, for example, green films which containa ceramic powder, an organic binder and a solvent are produced by filmdrawing or film casting. On some of the green films, an electrode pasteis applied by means of screen printing in order to form the innerelectrodes 20. The green films are stacked above one another along alength direction and compressed. The intermediate products of thecomponents are separated in the desired shape from the film stack.Lastly, the stack of piezoelectric green films and electrode layers issintered.

Outer electrodes 30, which are also shown in FIG. 1, are furthermoreapplied after the sintering.

In the embodiment shown here, the outer electrodes 30 are arranged onopposite side faces of the stack 1 and extend in the form of stripsalong the stack direction. The outer electrodes 30 contain, for example,Ag or Cu and may be applied onto the stack 1 as a metal paste and burntin.

Alternately along the stack direction, the inner electrodes 20 are ledup to one of the outer electrodes 30 and separated from the second outerelectrode 30. In this way, the outer electrodes 30 are electricallyconnected to the inner electrodes 20 alternately along the stackdirection. In order to produce the electrical connection, a connectionelement (not shown here) may be applied onto the outer electrodes 30,for example by soldering.

FIG. 2a shows an enlarged detail of the schematic side view of themultilayer component.

The component expands in the length direction when a voltage is appliedbetween the outer electrodes 30. In a so-called active zone, in whichneighboring inner electrodes 20 overlap in the stack direction, anelectric field is set up when a voltage is applied to the outerelectrodes 30, so that the piezoelectric layers 10 expand in the lengthdirection. In inactive zones, in which neighboring electrode layers 20do not overlap, the piezo actuator expands only slightly.

Owing to the different expansion of the component in the active andinactive zones, mechanical stresses occur in the stack 1. Such stressescan lead to poling and/or relaxation cracks 25 in the stack 1.

FIG. 2a shows a detail of a stack 1 of piezoelectric layers 10 and innerelectrodes 20, in which a crack 25 has been formed in the stack 1. Thecrack 25 extends parallel to the inner electrodes 20 inside the inactivezone, changes direction at the transition into the active zone andextends through neighboring inner electrodes 20 of different polarity inthe active zone. This can lead to a short circuit of the innerelectrodes 20.

FIG. 2b shows a detail of a stack 1 of piezoelectric layers 10 and innerelectrodes 20, in which a crack 25 has likewise been formed. Here, thecrack 25 extends parallel to the inner electrodes 20. With such aprofile of cracks 25, the risk of short circuits is reduced.

In order to promote such a profile of cracks 25, at least one layer ofelectrode material is provided with a PbO coating during the productionof the multilayer component. The PbO may be applied in the form of PbOpowder, Pb₃O₄ or another material containing PbO. Liquid forms ofchemical compounds containing Pb are likewise possible. The grain sizeof the PbO may have a median value d₅₀ (the particle size distribution)of from 0.1 μm to 2 μm, preferably from 0.3 μm to 1.5 μm.

In addition or as an alternative, PbO may also be mixed with theelectrode material. In this case, the PbO content relative to the sum ofPbO and the other metals of the electrode material is up to 100 wt %,preferably up to 50 wt %.

A layer of electrode material may be provided with the coating orcontain PbO; preferably, all layers may have a coating or contain PbO.

An intermediate phase, which contains PbPdO₂ and is spatially restrictedto the region in which the PbO was present, is then formed during thesintering. Owing to the different volume and the different expansionbehavior of this intermediate phase and of the contiguous piezoelectriclayer, microcracks are formed in the region of the intermediate phase.After the conclusion of the sintering, when the PbPdO₂ has beenreconverted, the region weakened by microcracks remains between theinner electrode and the piezoelectric layer. Relaxation and/or polingcracks, which are likewise spatially restricted, can then be formed fromthese microcracks. Branching of these cracks is thereby reduced orprevented.

FIG. 3a shows another detail of a schematic side view of the multilayercomponent. Here again, a crack 25 has been formed along an innerelectrode 20. The region 21, in which the crack is spatially restricted,is additionally shown.

This region 21 lies between the inner electrode 20 and the piezoelectriclayer 10, specifically where the coating containing PbO was arranged onthe electrode paste during the production method. In this region, thePbO reacted with the Pd from the electrode material to form PbPdO₂, andthereby formed the intermediate phase. Microcracks were able to form inthe intermediate phase, as described above, and opened to form cracksduring the polarization. The crack can only propagate in this region 21,so that it does not branch.

FIG. 3b shows a similar detail to FIG. 3a of a schematic side view ofthe multilayer component. Here, the cracks 25 extend around the innerelectrodes 20. Such a distribution may occur above all when the PbO hasbeen mixed with the electrode material, so that the region in whichPbPdO₂ is formed occurs uniformly around the electrode layer.

A multilayer component produced in this way may contain pure leadzirconate titanate (PZT) or a PZT modified with dopants.

An example of a multilayer component comprises the piezoelectric layer10 (Pb_(1−x+a),Nd_(x))((Zr_(1−z),Ti_(z))_(1−y),Ni_(y))O₃ as PZT ceramic.In this case, the values may be x=0.0001 to 0.06, a=0 to 0.05, z=0.35 to0.60 and y=0.0001 to 0.06. A PbO powder, which has a grain size of 1 μm,is used as a coating for the electrode layer. The inner electrodescontain Cu and Pd and a proportion by weight of PbO of from 10% to 50%in the electrode material. All electrode layers are printed with PbO forthe production method.

By the description with the aid of the exemplary embodiments, theinvention is not restricted thereto but covers any new feature and anycombination of features. This in particular comprises any combination offeatures in the patent claims, even if this feature or this combinationis not itself indicated explicitly in the patent claims or exemplaryembodiments.

What is claimed is:
 1. A piezoelectric multilayer component comprising:a stack of sintered piezoelectric layers and inner electrodes arrangedbetween the piezoelectric layers, wherein a region that comprises polingcracks is present on a surface of at least one of the inner electrodes;and wherein the region that comprises poling cracks has at least one ofa PbO content that is greater than a PbO content of the piezoelectriclayers, and traces of PbPdO₂.
 2. The multilayer component according toclaim 1, wherein the poling cracks are substantially free of branching.3. The multilayer component according to claim 1, wherein the innerelectrodes comprise Pd.
 4. The multilayer component according to claim1, wherein the piezoelectric layers comprise lead zirconate titanate.